<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <div class="parent">
    <div class="left">aaa</div>
    <div class='right'>bbb</div>
  </div>

</body>

<style>
  .parent {
    display: flex;
    flex-direction: row;
    flex-wrap: nowrap;
    width: 500px;
    justify-content: space-between;
    background-color: blanchedalmond;
  }

  .left {
    height: 200px;
    width: 200px;
    background-color: aquamarine;
    flex-grow: 1;
  }

  .right {
    height: 200px;
    width: 200px;
    background-color: black;
  }
</style>

</html>